In recent years, function/structure analysis of protein produced using genetic information or biopolymer peptide post-translationally modified and functioning in cells based on the protein has attracted attention.
Mass spectrometry has been attracting attention as means for such function/structure analysis. By use of the mass spectrometry, it is possible to obtain sequence information of protein or a peptide component as a biopolymer component in which amino acids are linked by peptide bond. Particularly in a mass spectrometer with an ion trap using a high frequency electric field, MSn measurement can be performed in an ion trap section, as disclosed in Patent Literature 1.
A sample is ionized in an ionization section, and then introduced and accumulated in the ion trap section. Next, parent ions are isolated by use of FNF (Filtered Noise Field). Next, CID (Collusion Induced Dissociation) is set up, and dissociated ions are detected by an ion detection section to obtain MSn spectra. Such an ion trap or TOF (Time Of Flight) type mass spectrometry, which can achieve high-speed analysis, has high compatibility to a sample separating method such as liquid chromatography. Accordingly, the ion trap or TOF type mass spectrometry has been used widely in analyses such as proteome analysis where continuous analysis of a sample is regarded as important.
Currently, the aforementioned CID is the most widely used method in the field of protein/peptide analysis. When a peptide consisting of amino acids is dissociated using this method, the peptide is preferentially dissociated in portions attributed to a-x and b-y. However, some amino acid sequence has a portion which may be difficult to dissociate. In addition, when ion dissociation is performed by CID, a post-translationally modified peptide or the like has a tendency that side chains produced in the post-translational modification are cut easily. As a result, a modification molecular species and presence/absence of modification can be confirmed from detected ions, but it is difficult to determine the portions where amino acids have been modified.
On the other hand, ECD (Electron Capture Dissociation) is attracting attention as another dissociation means in the field of protein/peptide analysis. Using ECD, one c-z portion on a main chain of amino acid sequence is cut off without depending on the amino acid sequence (provided that any proline residue with a cyclic structure is not cut off exceptionally). As a result, amino acid sequence, a post-translationally modified molecular species, and a modified portion can be analyzed perfectly only by mass spectrometry.
In recent years, a mass spectrometer in which ECD can be performed in an ion trap section has been developed as disclosed in Patent Literature 2. Such an apparatus has been attracting attention because CID measurement and ECD measurement can be performed by one apparatus so as to acquire a large amount of analysis information about biopolymers. Because compatibility to liquid chromatography is good, it is therefore important to perform ECD protein/peptide analysis at a high speed.
In the field of protein/peptide structure analysis, in order to acquire spectra useful for the structure analysis, it is important to detect a large number of fragment ions resulting from the structure with high efficiency and with high sensitivity.
Typically in CID generally used as structure analysis of peptide, only parent ions are dissociated by collision. It is therefore important to dissociate as many parent ions as possible in order to increase the signal intensity of fragment ions. To this end, various methods for adjusting CID time, CID voltage, etc. in real time have been invented and put to practical use. Also in ECD, it is important to dissociate as many parent ions as possible.